In a power conversion system such as a variable-speed, constant-frequency (VSCF) power generating system, a brushless, three-phase synchronous generator converts variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.
Prior VSCF systems have utilized either a pulse-width modulated (PWM) inverter or an inverter of the stepped waveform type. In either case, in order to obtain sufficient voltage and current handling capabilities, it has been found necessary to connect power switches in parallel to the load. As is generally known, operating semiconductor switching devices in parallel presents significant difficulties. Specifically, the characteristics of these devices must be carefully matched to insure that the currents handled by the devices are shared as equally as possible so that the potential for damage or destruction of the switches is minimized.
Inverters have been designed wherein first and second power switches are connected to end terminals of an interphase transformer having a mid-tap at which an output is produced. The interphase transformer is designed so that the output voltage at the mid-tap is at a level intermediate the voltages applied by the switches. In this type of inverter, each switch handles only a portion of the total output current magnitude. Patents disclosing such inverters include Urish, U.S. Pat. No. 3,657,633, Heintze, U.S. Pat. No. 3,943,429, Honbu, et al., U.S. Pat. No. 4,549,258 and Mizoguchi, U.S. Pat. No. 4,802,079.
Glennon, U.S. Pat. No. 5,008,801 discloses a VSCF power conversion system using an output autotransformer having first and second winding portions coupled to first and second switches, respectively, wherein the first and second winding portions are connected together at a common junction and wherein a secondary winding is connected between the junction and a particular potential. The first and second switches are operated in unison and the first and second windings act like an interphase transformer to balance the currents flowing in the associated switches.
While interphase transformers are effective to balance currents flowing through parallel connected switches, such reactors permit DC content to reach loads connected thereto. Such DC content may arise, for example, from asymmetrical switching of the inverter switches. This DC content is objectionable and should be eliminated, if possible. Systems and methods have been developed to eliminate DC content in the output of an inverter, such as the system disclosed in Roe, et al., U.S. Pat. No. 4,882,120; however, these types of systems introduce complexity into the inverter control since such systems rely upon adjustment of the operation of the inverter switches to eliminate this DC content.